Thermionic tube



Dec. l2, 1939.

H. W. PARKER Z,83,l49

THERMIONIC TUBE Filed April 26, 1958 INVENTOR HE/vm/ W PAR/ 5:

ATTORNEY lli Patented Dec. 12, 1939 S'E'ATES tisana THERMIONIC TUBEHenry W. Parker, rEoronto, Ontario, Canada, as-

signor to Rogersadio Tribes Ltd., Toronto, Ontario, Canada, acorporation of' Ontario,

Canada Application ApriirZ, 1938, Serial No. 204,264

1 Claim.

My invention pertains to thermionic tubes and relates more especially tothat type of such tubes empioying screen grids and known as'output powerpentodes.

Output power pentodes as now constituted often exhibit a defect known asblocking when the tube is subjected to Very strong signals entering thepositive grid region. A

It is well known that a high speed electron on hitting a solid substancesplashes out other electrons known as secondary electrons which reboundwith less velocity than the pim. ary electrons and also that a primaryelec-tron can splash out several secondary electrons depending upon thework function of the surface upon which the primary electron impinges.With an oxide coated cathode the control grid due to contamination fromthe cathode has a very low work function surface and there is an' efect,due to secondary emission from the grid, which causes a very low valueof negative grid resistance in the positive grid region. This negativegrid resistance or slope causes trouble when the grid is subjected to astrong signal before the cathode associated therewith is fully heatedup. The cathodes of a power output tube heat up slower due to theirgreater mass than the cath'odes of the preceding amplier tubesin aplural stage ampliiier so that often a strong signal is put on the powertube grid before the cathode thereof is heated up sufficiently. rl'hisswings the control grid of the power output tube into the positive gridregion and once this region .is entered the grid potential remainspositive. Sometimes it is approxmately one-half the anode potential andthisl high positive potential of the grid draws saturation current fromthe cathode which is limited only by emission from the cathode. Suchhigh current overheats ythe tube, liberates gas and destroys thefunction of the tube. This occurrence is termed blocking The tube firstoverheats, causing the liberation of gas, and this liberation of gascauses loss of grid bias which causes increase of space current whichmay finally result in the destruction of the tube.

Several methods of overcoming this defect have been employed, such asgraphitizing the control grid, oxidizing the grid, or utilizing aconstruction in which the grid rods of the control grid are of largediameter and placed relatively close to the cathode. -All of theseexpeable disadvantages.

(Cl. Z50-4.76)

One of the objects contemplated by my invention comprises producing apower output pentode which will not be subject to the above noteddefect.

Another object contemplated by my invention comprises producing a poweroutput pentode employing means in association with the control grid forpresenting at all times during the operation of the tube a positive gridresistance to overcome the defects above noted.

A still further object comprises producing an output power pentodehaving simple and effective means for overcoming the defect above notedwithout introducing other undesirable electrical or mechanical effects.

I accomplish all of the above noted Adesirable results'and others whichwill hereinafter become apparent by means of the novel construction,combination, arrangement and interaction of parts which will behereafter more speciiically described with reference to the accompanyingdrawing forming a part of this specification and in which like referencenumerals designate corresponding parts throughout.

Inthe drawing:

Fig. 1 isa broken, partially sectioned view of my improved tube;

Fig. 2 is a sectional View of the' same taken on the line 2 2 of Fig. 1;and

Fig. 3 is a set of curves showing the characteristics of a power outputpentode employing my improvement.

Referring now especially to Figs. l. and'2 which illustrate a powerpentode incorporating my improvement, and with the usual base andcontact prongs omitted, the envelope Il which may be of metal but ispreferably of glass or other vitreous material, supports the mual stemand press 5 lwhich in turn support the electrodes. The anode 6 issupported by the usual standards and Within the anode are supported theouter screen grid "I, inner screen grid 8, control grid 9 and indirectlyheated cathode I0. The usual oontactleads for the electrodes are shownin dotted line in the press and stem. The Contact lead for the cathodeincludes the metallic support I5 and link I4, while the Contact lead forthe outer screen grid 'l includes the link iB. The usual metallicradiating vane H is welded to the control grid standards in order toreduce the temperature of the control grid to eliminate primary electronemission therefrom. My improvement comprises a small metallic targetwhich may be termed a diode rectifier or diode target I2 and which inthe embodiment illusenlarged trated is welded as shown in Fig. 2` to thewires of the control grid 9. It is thus maintained in relatively closeproximity to the cathode lil. For purposes of clarity, only a few of theconvolutions of the wires composing the screen grids 'l' and 8 areshown.

The diode target, while shown in the form or a disc, may be a smallsquare of nickel, molybdenum or other refractory metal and may be of anysuitable size, shape or form. The area of the diode target should be ofsuch dimensions that it will not interfere with the characteristics ofthe tube as far as plate transconductance and gain is concerned and forthis reason the diameter or area of the diode disc is made rather small.If the area is made too large, it would interfere with gaincharacteristic of the tube, and if it is too small it would not functionto present a posi tive grid resistance. The diode is placed close enoughto the cathode and of such dimensions that the eld of the anode isprevented from extending between the target and the cathode to influencethe iiow of electrons to the target and thus any secondary electronsreturn to the target instead of being drawn to the plate. Practicallyall of the secondary electrons derived from the target return to thetarget and hence there are always more electrons arriving at the grid towhich the target is attached than are leaving the grid. Incrementally,this means that at no time will the grid have a negative slope but thederivative of the grid characteristic will always have a positiveresistance value.

In power pentodes of the standard type incorporating my invention whichI have constructed and successfully demonstrated, I have found that anickel target of approximately 1/3 diameter` united to the outside ofcontrol grid as shown in Figs. l and 2 operates very effectively toovercome the difficulty of blocking and yet does not interfere with anyof the desired electrical characteristics of such tube. The diode targetis in reality a separate entity from the grid and it could be a separatediode connected to the grid externally of the tube. Obviously, there isan optimum size for the target, not too large and not too small, whichwill present a positive grid resistance with minimum reduction in gaindue to the projected area of the diode cutting down the eifective areaof the anode.

The operation of the diode target is shown in Fig. 3, in which curve lshows the diode target characteristics alone. This shows that there ispresented a low positive resistance because the plate field cannot reachin behind and between the target and the cathode to inuence secondaryelectrons from returning to the target. Curve 2 illustrates the gridwire characteristics alone,

while curve 3 represents an addition of curves l and 2 and illustratesthe characteristics of the combined diode target electrically associatedwith the grid wire. In effect, this electrical association is a parallelconnection. The region A in the curve is a positive grid region andextends normally to a value of about 20 volts. The region B is apositive region and extends normally from plus 2O volts to aboutone-half anode voltage, whereas the region C is a positive grid regionand extends normally from about one-half anode voltage to full platevoltage. Referring to the curves it will be noted that the region B oncurve 2 shows a negative slope. This negative slope is one reason forthe difficulty known as blocking and when blocking occurs, the grid isthrown over into the region B. Once this region B is entered, the gridpotential remains in the bottom of the valley indicated on curve 2between the regions B and C. It happens that the grid potential in thisregion is approximately onehalf of the plate potential and is positiveand the grid will remain at a high positive value drawing saturationcurrent from the cathode. As such current is limited only by theemission from the cathode this high current is suflicient to overheatthe tube, liberate gas, and destroy the function ci the tube, finallycausing the destruction of the tube.

It will be apparent from the foregoing that my improved power pentodeconstruction is completely eective in eliminating the difculty known asblocking; that the means employed are simple and completely effectiveand that the means employed does not introduce any unwanted electricalor mechanical effects.

While I have thus illustrated and described one complete embodiment ofmy invention, it will be apparent that many changes can be made there inwithout departing from the intended scope and spirit of the invention. Ido not therefore desire to limit myself to the foregoing except as maybe pointed out in the appended claim in which I claim and desire tosecure by United States Letters Patent the following:

A thermionic tube comprising, a substantially evacuated envelopeenclosing a cathode, an anode, a control grid, and an additionalelectrode conductively connected to said control grid for neutralizingthe effect of secondary emission from said control grid, said electrodecomprising a small metallic target positioned adjacent one side only ofsaid grid, between said cathode and said anode, in close proximity tosaid cathode and having a projected area considerably less than thetotal area of that side of said control grid to which it is adjacent. v

HENRY W. PARKER.

